Abstract. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on
ENVISAT has made extensive measurements of polar stratospheric clouds (PSCs)
in the northern hemisphere winter 2002/2003. A PSC detection method based on a
ratio of radiances (the cloud index) has been implemented for MIPAS and is
validated in this study with respect to ground-based lidar and space borne
occultation measurements. A very good correspondence in PSC sighting and
cloud altitude between MIPAS detections and those of other instruments is
found for cloud index values of less than four. Comparisons with data from the
Stratospheric Aerosol and Gas Experiment (SAGE) III are used to further show
that the sensitivity of the MIPAS detection method for this threshold value
of cloud index is approximately equivalent to an extinction limit of
10-3km-1 at 1022nm, a wavelength used by solar occultation
experiments. The MIPAS cloud index data are subsequently used to examine,
for the first time with any technique, the evolution of PSCs throughout the
Arctic polar vortex up to a latitude close to 90° north on a near-daily
basis.
We find that the winter of 2002/2003 is characterised by three phases of very
different PSC activity. First, an unusual, extremely cold phase in the first
three weeks of December resulted in high PSC occurrence rates. This was
followed by a second phase of only moderate PSC activity from 5-13 January,
separated from the first phase by a minor warming event. Finally there was a
third phase from February to the end of March where only sporadic and mostly
weak PSC events took place. The composition of PSCs during the winter period
has also been examined, exploiting in particular an infra-red spectral
signature which is probably characteristic of NAT. The MIPAS observations
show the presence of these particles on a number of occasions in December
but very rarely in January. The PSC type differentiation from MIPAS
indicates that future comparisons of PSC observations with microphysical and
denitrification models might be revealing about aspects of solid particle
existence and location.